1. Field of the Invention
The present invention relates to an image coding technology and an image decoding technology, and it particularly relates to method and apparatus for preferentially coding or decoding a partial region of an image. The present invention further relates to an image display apparatus and an image displaying method, and relates also to apparatus for reproducing image streams.
2. Description of the Related Art
At ISO/ITU-T, JPEG2000 using a discrete wavelet transform (DWT) is being standardized as a successor to JPEG (Joint Photographic Expert Group), which is a standard technology for compression and coding of still images. In JPEG2000, a wide range of image quality, from low bit-rate coding to lossless compression, can be coded highly efficiently, and a scalability function, in which the image quality is gradually raised, can be realized easily. Moreover, JPEG2000 comes with a variety of functions which the conventional JPEG standard did not have.
As one of the functions of JPEG2000, the ROI (Region-of-Interest) coding is standardized, in which a region of interest of an image is coded and transferred in preference to other regions. Because of the ROI coding, when the coding rate has an upper limit, the reproduced image quality of a region of interest can be raised preferentially, and also when a codestream is decoded in sequence, a region of interest can be reproduced earlier with high quality.
As the ROI coding, the MAXSHIFT method scales up the bit-planes of wavelet transform coefficients corresponding to a region of interest in an image (hereinafter referred to as “ROI transform coefficients”) by a maximum number of bits of the bit-plane of wavelet transform coefficients corresponding to non-ROI regions (hereinafter referred to as “non-ROI transform coefficients”). According to this technique, all the bit-planes of ROI transform coefficients are coded earlier than any of the bit-planes of non-ROI transform coefficients. A method is also known, in which the high-order bits in part of the ROI transform coefficients are coded in preference to the non-ROI transform coefficients by scaling-up the bit-planes of ROI transform coefficients by a predetermined number of bits. Reference (1) in the following Related Art List discloses an improvement technology for these ROI coding techniques.
FIG. 1 illustrates a structure of a conventional image coding apparatus 100. This image coding apparatus 100 carries out a ROI coding by the MAXSHIFT method. A wavelet transform unit 10 performs a wavelet transform on an inputted original image and outputs the wavelet transform coefficients. A quantization unit 12 quantizes the wavelet transform coefficients of the original image. FIG. 2A shows wavelet transform coefficients 50 after quantization, which include bit-planes each having five bits from MSB (most significant bit) to LSB (least significant bit).
A ROI selector 18 selects a region of interest (ROI) in an original image, and a ROI mask generator 20 generates ROI masks that are used to specify the ROI transform coefficients, which are the wavelet transform coefficients corresponding to the selected region of interest. In FIG. 2A, the ROI transform coefficients are represented by the shaded portions of the wavelet transform coefficients 50.
A ROI scale-up unit 22 scales up the quantized ROI transform coefficients by S bits by referring to the ROI mask. That is, the values of the ROI transform coefficients are shifted to the left by as much as S bits. Here, the amount S of the scale-up is a natural number larger than the number of bits of the maximum value of the quantized values of non-ROI transform coefficients, which are the wavelet transform coefficients corresponding to the regions outside the region of interest. FIG. 2B shows wavelet transform coefficients 52 with the ROI transform coefficients scaled up by 5 bits. After the upscaling, zeros are assigned to the digits that are newly created as a result of the upscaling in the wavelet transform coefficients 52.
An entropy coding unit 14 entropy-codes the quantized values of the wavelet transform coefficients 52 resulting from a scale-up by scanning them in order from high-order bit-planes as indicated by the arrows in FIG. 2C. A coded data generator 16 turns the entropy-coded data, together with the coding parameters such as quantizing width, the ROI position information indicative of the position of a region of interest and the amount of scale-up, into a stream and outputs it as a coded image.
FIG. 3 illustrates a structure of a conventional image decoding apparatus 110. This image decoding apparatus 110 decodes images which are ROI-coded by the MAXSHIFT method. A coded data extracting unit 30 extracts coded data, various types of coding parameters, ROI position information and scale-up amount from the inputted coded image. An entropy decoding unit 32 decodes the coded data bit-plane by bit-plane and stores in a memory the derived quantized values of the wavelet transform coefficients. FIG. 4A shows wavelet transform coefficients 54 after an entropy decoding. In correspondence to the example of FIGS. 2A to 2C, the amount S of scale-up is 5 bits, and the wavelet transform coefficients 54 after the entropy decoding include a total of 10 bit-planes.
A ROI mask generator 38 generates masks that specify ROI transform-coefficients based on the ROI position information. A ROI scale-down unit 40 scales down, by S bits, the ROI transform coefficients out of the quantized wavelet transform coefficients by referring to the ROI mask. FIG. 4B shows wavelet transform coefficients 56 after the ROI transform coefficients have been scaled down by 5 bits. The 5 high-order bits of non-ROI transform coefficients left by the scale-down are discarded, and as a result, a total of 5 bit-planes of quantized wavelet transform coefficients 56 are obtained.
An inverse quantization unit 34 inverse-quantizes the quantized values of the wavelet transform coefficients including the scaled-down ROI transform coefficients, and an inverse wavelet transform unit 36 performs an inverse wavelet transform on the inverse-quantized wavelet transform coefficients and outputs a decoded image thus obtained.
Motion-JPEG2000, when compared with MPEG (Moving Picture Experts Group) and other moving image coding specifications, is inferior in compression ratio because of its inability to use frame correlation, but is superior in its capacity for editing in frame units and reversible coding, thus holding numerous potentialities.
As an example of a technology utilizing these characteristics of JPEG2000, Reference (2) in the following Related Art List discloses a technology for simplifying the decoding processing, as necessary, by comparing elapsed time against time limit in each stage of image decoding. According to the technology, a relatively small-scale structure may realize image decoding with relatively high image quality.
Related Art List
    (1) Japanese Patent Application Laid-Open No. 2001-45484.    (2) Japanese Patent Application Laid-Open No. 2002-325257.
In the above-mentioned conventional ROI coding, coding of the bit-planes is performed after the scale-up of the wavelet transform coefficients corresponding to a region of interest, so that an extra memory capacity to store the bit-planes after the scale-up is required. Moreover, the coding processing takes a longer time because an excess coding must be carried out for the scaled-up bit-planes.
Also, in the decoding of a coded image after a ROI coding, the coded image data are decoded for each bit-plane, so that an extra memory capacity is required in response to the amount of scale-up. Moreover, this decoding process requires specifying wavelet transform coefficients corresponding to the region of interest by acquiring ROI position information from the coded image data and scaling down the thus specified wavelet transform coefficients of the region of interest.
As described above, the conventional ROI coding has problem and drawback in that it involves a scale-up processing, which requires an excess memory capacity and an eventual increase in the amount of computation.
The conventional ROI coding as described above also has a problem in that when there are a plurality of regions of interest, it cannot provide any difference in image quality among the plurality of regions of interest.
It is expected that distribution and use of image contents utilizing JPEG2000 will increase drastically in the years to come. Among such images to be distributed, however, there may be images, like those of a surveillance camera, which requires higher image quality for a certain part of them and not so high quality for the remaining part of them. Furthermore, when a region specified by the user is set for a higher image quality, there may be cases where the image processing capacity is exceeded due to the increase in the amount of processing, thus resulting in the drop of frames.